I know how to read a data when "f " record contains more than 3 group of indices. For processing a data I use following algorithm:

* If that set of indices has not been seen before:
** Add the vertex attributes referenced by that index to the attribute arrays.
** Add the index of the new vertex to the index buffer.
** Store the fact that this particular set of indices has been seen before, and which index you used in the index buffer with it.
* If that set of indices has been seen before:
** Retrieve the index for the index buffer that was stored for this set of indices.
** Add that index to the index buffer.
** Do not add any vertex attributes to the attribute arrays

The above obj file contains vn records and the algorithm process them properly so how should I interpret smoothing groups ? Are they necessary in that case ?

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Normal groups are very useful. Say if you render a cube you don't want to normals to be smoothed on the shared vertices for example. You would want it to look like flat shading and so have 6 normal groups ......